1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for reducing drag produced by a streamer spread when used with a seismic acquisition system.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which is especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system for generating seismic waves and recording their reflections off geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers 12 are attached to the vessel 10 with a front-end gear 13. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface 14, or they may have other than horizontal spatial arrangements. The vessel 10 also tows a seismic source array 16 configured to generate a seismic wave 18. The seismic wave 18 propagates downward and penetrates the seafloor 20 until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver 11 on the streamer 12. Based on analyses of the data collected by the receiver 11, an image of the subsurface is generated.
A top view of half of the front-end gear 13 is illustrated in FIG. 2. As noted above, the front-end gear 13 connects plural streamers 12 to the vessel 10. The front-end gear 13 includes at least a wide-tow rope 30 for connecting a deflector 32 to the vessel 10. Another wide-tow rope (not shown) connects another deflector (not shown) to the vessel, and the two deflectors sandwich the heads of the streamers 12. The deflectors are configured to apply tension between the streamer heads so that the separation ropes 34 between the streamers are stretched and, thus, the cross-line distance between adjacent streamers is maintained constant.
Further, the front-end gear 13 includes lead-ins 401 connecting the head of each streamer to the vessel. While the wide-tow rope may be a rope, i.e., may not include any cable for transmitting electric power and/or data, the lead-ins include wires or cables with capabilities to transmit electric power from the vessel to the streamers, and data from the streamers to the vessel. Thus, the lead-ins are heavier than the wide-tow rope and produce considerable drag, especially those lead-ins connecting to the streamers farther away from the traveling direction 44. In this respect, FIG. 2 shows that lead-ins 40A-C make a much larger angle with the traveling direction 44 than lead-ins 401, consequently resulting in greater drag.
Thus, it is desirable to have a front-end gear with decreased drag produced by the lead-ins farthest away from the vessel's traveling direction.